The reasons for this are many, but the main thing for us is so that we can push updates to you without having to sell new versions every couple of years, and having to support the old versions people keep using when updates cost. This is why Adobe Photoshop and others have moved to this system. It is also generally cheaper for the users than buying a new version every two years or so. You can just pay for the months that you use it, and then stop when you are not.